Chemically obtained cellulose e.g. as it is obtained by the sulfite process or the alkaline sodium or sulfate process still contains besides the chief component cellulose small amount of lignin, hemicelluloses and several other constituents. The mentioned materials accompanying the cellulose, above all the lignin, cause the coloration of the cellulose or the products produced therefrom.
In order to produce paper or other products of high whiteness from the cellulose, which products are not inclined to yellow it is necessary to remove the remaining accompanying materials after the chemical decomposition.
It is known from German Pat. No. 2,219,505 (and related Hebbel U.S. Pat. No. 3,867,246) to bleach in a multiple step process with peroxide and peracid. Thereby the cellulose is bleached, in a given case after an acid pretreatment, in the first step with a peroxide, in the second step with an organic peracid and in the third step with a peroxide. Between the separate bleaching steps the cellulose is washed copiously with water.
A considerable disadvantge of this known process is that for the bleaching with peracid there must be employed an equilibrium peracetic acid. Then in the production of te equilibrium peracid a very high excess of carboxylic acid must be employed in order to shift the equuilibrium as far as possible to the side of peracid. The hydrogen peroxide not reacted to the peracid is lost, namely in the washing of the cellulose subsequent to the peracid bleaching step.
In the known process there is employed an equilibrium peracid in which there is used in its production an 8 to 10 fold excess of carboxylic acid.
Because of the amount of carboxylic acid necessary for this the known bleaching process is uneconomical.
To be sure a reduction of this excess saves carboxylic acid, but leads to a higher residual content of hydrogen peroxide and therefore to a likewise uneconomically high hydrogen peroxide requirement.
A further disadvantage of the process of German Pat. No. 2,219,505 (and the related Hebbel U.S. Pat. No. 3,867,246) is that in using the equilibrium peracid because of thhe slow establishment of the equilibrium large supply vessels are necessary. Thus an average cellulose factory already needs supply vessels having a volume up to 100 m.sup.3.
The alternative production of the peracid from carboxylic anhydride as well as the employment of pure peracid for reason of operational safety is not possible because of the dangerousness of concentrated organic per compounds.
Thus in the reaction of carboxylic anhydride with, e.g. hydrogen peroxide, there is formed in addition to the peracid also the dangerous diacylperoxide which is inclined to decompose spontaneously.
Also the production of the peracid from carboxylic acid anhydride and perroxide in the cellulose pulp itself can only be carried out with high peroxide losses because of the dilution which is then present.